'88 300E Octane Dial
 

10 or more years ago, I read about the resistor in the wiring harness near the master cylinder. The resistor is a signal to the EZ ignition control that determines spark advance. I learned that removing the resistor offered maximum spark advance, thus performance and economy gains if using high octane fuel.

I removed the resistor at that time, given that Premium was probably a $1.30 at the time.

Since putting my 300E back on the road as a work car and given TODAY's fuel prices, I put the resistor back in place and have been running with no detonation on regular gas.

I remember reading at that time that on some cars delievered in parts of the world other than the US, a dial was installed that basically placed different resistances at that location in the circuit based on dial setting. This allowed for operation in parts of the world where octane availability was quite low.

Does anyone know anything about this dial? If a person knew the particular resistances involved, it wouldn't be difficult to fabricate such a dial mechanism.

Thanks for sharing any knowledge you might have.

I have that dial sitting in my garage. It basically is an 8 position switch where you can set from 0 retard, to lots of retard.

Ill try and snap a pic of it later.

Did we ever get a pic or more information on this?? Curious how the cars handle regular unleaded, as the manual says not to go over 2000 rpm or more than 1/3 throttle IIRC.

we run regular in our 103's and 104 with no loss of power, no detonation, and no change in mileage vs premium.

it might be different if you live in mountainous areas

If the car is running near stoich, it will detonate near the top of the revs.

If you drive the car slowly, and keep it farrrrr away from the redline, 87 is ok.

I totally forgot to post a picture. Ill try and remember (again) this evening

Just worth noting, 30 cents extra a gallon for premium fuel only equates to an extra.....

5 dollars a tank.

Ah, Eugene; yes, 30 cents a gallon sounds very small, but when amortized out over 20 years and 200k miles you'll save over $3000. My 20-year-old car is now worth maybe $3000; so, I have a free S-class Mercedes to drive for the next few years. Maybe old Ben Franklin was on to something.

Gas prices are an emotional thing; folks will drive all over to save a nickel per gallon, then walk out of the house and leave 50 lights on.

But, I do agree that if you plan to run her a bit hard in hot weather, then the possibility of High-RPM knock is there. The one and only time I ever heard knock on 87 octane was one very hot day when pulling past 5000 RPM on an incline. Haven't had the occasion to do that again in the last ten years or so.

DG

I agree, however, there are a million other things one could do to save 30 cents other than getting chincy on the premium fuel. The car calls for premium, so why not give it?

Quit going to starbucks, quit smoking, fast food, candy, etc..

I can go on and on about saving (according to your figures) $12.50 a month.

Risking detonation on my mercedes?. No thanks. If your car does not show detonation at 4+k rpms with 87 octane, your engine is running too rich, and is wasting gas.

no starbucks, etc.... i do my own work...and drive my car normally... if i ever heard detonation , i would move up...

the s320 runs perfectly on regular fuel, btw

http://farm8.staticflickr.com/7091/6...37cecfeb_b.jpg

http://farm1.staticflickr.com/199/49...171_z.jpg?zz=1
http://farm4.staticflickr.com/3180/3...0ea_z.jpg?zz=1

Every situation is different. Altitude in my area is about 1400ft. Hot summers - relatively mild winters.

My M103 motor seems to be more robust on 93 octane fuel.

As stated above, other needless habits can be eliminated in order to feed my MB what she likes.

Just to add to the equation, the 84 and 85 117 engines (maybe others) call for 87 oct. I used premium fuel for years and the car ran good. When gas went over $4 one time, I switch to reg. Guess what, I could feel that the car actually ran better. Fuel mileage was up a little also. Now to go deeper, the engine came out of a car made for the Mexican market (long story). The CR is 8.0 to 1 as far as I can find out, while the US version is 9.0 to 1. So what I have concluded, the German car makers know what they are doing. If they say Premium, use it. If they say Reg, use it.

Paul